Electron spin resonance of separation by implanted
Evidence for structural change and a deep electron
oxygen
trap
oxides:
J. F. Conley, Jr. and P. M. Lenahan
Pennsylvania State University, University Park, Pennsylvania 16802
P. Roitman
National Institute of Standards and Technology, Gaithersburg, Maryland
(Received 27 December 1991; accepted for publication 2 April
1992)
We present direct evidence for deep electron traps and structural changes in separation by
implanted oxygen (SIMOX) buried oxides and evidence that some positively charged E’ centers
are compensated by negatively charged centers in SIMOX oxides.
Separation by implantation of oxygen (SIMOX) is the
leading technology for silicon-on-insulator ( SOI), a device
fabrication method with great promise for use in satellites
and ultralarge scale integration. SIMOX buried oxides
contain trapping centers that may play a significant role in
the operation of devices utilizing this technology. We study
trapping centers in SIMOX buried oxides with a combination of electrical measurements and electron spin resonance (ESR) .I
Combining ESR and capacitance vs voltage (CV)
measurements, we recently found that very high densities
( - 10i8/cm3) of paramagnetic (ESR active) point defects
called Ev centers are generated in SIMOX buried oxides
exposed to vacuum ultraviolet (WV)
irradiation (AC//~
210.2 eV).25 This indicated the presence of a very high
density of E precursors in the buried oxides. The E’ center
is an unpaired spin on a silicon bonded to three oxygens;
the E’ ESR spectrum has a zero crossing g-2.0005. We
searched for other ESR spectra in the vicinity of g=2.000
but have not yet been able to detect other signals including
the “amorphous silicon” signal reported by Stessmans,
Revesz, and Devine.”
The creation of high densities of E centers was accompanied by virtually no net space charge in the buried
oxide.39J7 This absence of net space charge in the presence
of a large E density suggests two possibilities:3-5Y7 ( 1) that
the E’ centers are neutral, or (2) that the E’ centers are
positively charged (E’ centers are the dominant deep hole
trap in thermal oxides)’ and compensated by negatively
charged centers.3-5’7 To test these possibilities and determine whether or not SIMOX E centers are electrically
active, we performed a series of charge injection
experiments.2-5*7 Injection of electrons into VUV illuminated oxides reduced E’ amplitude; injection of holes into
the oxides increased E” amplitude.4’5*7 Both of these results
demonstrate that SIMOX E’ centers are electrically active
and that at least some of them are positively charged when
paramagnetic. However, in these experiments we consistently observed an E’ density greater than total charge
density.4*5.7The fact that the trapped charge density is considerably lower than the presumably positive E’ center density leads one to suspect some form of electrically compensating mechanism to account for the discrepancy.
In this letter, we determine more directly the effects of
electron and hole injection on the buried oxide and provide
2889
Appl. Phys. Lett. 60 (23), 8 June 1992
very strong evidence for compensating positive and negative charge. We also provide evidence for structural
changes and the creation of a deep electron trap as a result
of VUV illumination.
The samples used in this study include P( 100) 405 nm
single implant and N( 100) 385 nm multiple implant SIMOX oxide samples. Both single and multiple implant
samples received a 5 h anneal in 99.5% argon and 0.5%
oxygen at 1315 “C. All samples received a total oxygen
dose of 1.8 X 10”/cm3 at an energy of 200 keV, a current of
34 mA, and an implant temperature of 640 “C. A residual
oxide and the top layer of silicon were removed by subsequent etches in HF and then KOH at room temperature.
The behaviors of the multiple and single implant oxides
were qualitatively the same though not identical.
We made X-band ESR measurements at room temperature using a TE,,, “double” resonant cavity and a “weakpitch” spin standard. Relative spin-concentration measurements are accurate to f 10% with an absolute accuracy of
a factor of two. High frequency CV measurements were
taken at room temperature using a mercury probe. Net
oxide space charge density was determined from CV curve
shifts. (Etchback experiments indicate charge trapping
throughout the oxides).
E’ centers were generated by exposing (bare) buried
oxides to VUV light from a 50 W deuterium lamp in a
vacuum. In some cases, a filter passing only 10.2 eV photons was used; in these cases the oxides were illuminated
briefly under positive bias. Biasing was performed by depositing low-energy ions created by corona discharge’ onto
the samples. Surface potentials were measured with a
Kelvin probe electrostatic voltmeter. [Most of these
10.2 eV photons are absorbed in the top 100 A of the oxide
where they create electron hole pairs.‘oP” The positive bias
drives holes across the oxide (hole injection) while sweeping electrons out.] In other cases, the oxides were VUV
illuminated unbiased without the filter (he/A< 10.2 eV) for
an extended period ( -40 h). Exposing the samples to this
“extended” VUV illumination generates extremely high
densities of paramagnetic E’ centers ( - 10*8/cm3).
Ultraviolet illumination (UV) from a sub-SiOl band
gap (he/A. = 5.5 eV) mercury-xenon lamp was also used in
combination with positive corona bias. The UV illumination results in the internal photoemission of electrons from
0003-6951/92/232889-03503.00
@ 1992 American Institute of Physics
2889
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Post electron
injection
i--xl- /c--M~
Post h o l e
injection
Post 4 0 h o u r
VW
.
PRE
VUV
El
Hi
3.2 G
1-.-
E2
H2
E3
b) E S R D A T A
I-- -- ~ FIG. 2. R S R traces of S IM O X oxide (a) after the injection of about lOI
electrons/cm2, (b) after the injection of about lOI holes/cm”, a n d (c)
after e x p o s u r eto 4 0 h of V U V illumination. In all traces, s a m p l e size a n d
g e o m e t r y w e r e identical. Spectrometer settings w e r e identical in traces
(a) a n d (b) but g a i n w a s r e d u c e d in trace (c).
--i
PRE
VUV
I.
El
HI
_I.^_.“_-.i-....E2
H2
E3
FIG. 1. E ffects of electron ( E ) a n d h o l e (H) injection o n (a) s p a c e
c h a r g e density ( C V ) a n d (b) spin density ( E S R ) m e a s u r e m e n t sof W V
illuminated buried oxides.
the Si. T h e positive bias drives electrons across the b u r i e d
oxide (electron injection).
Figure 1 s h o w s quantitative results of electron/hole
c h a r g e cycling o n W V irradiated single implant S IM O X
oxides. Consistent with earlier work,3-5*7the 4 0 h of V U V
illumination results in a large E ’ signal [Fig. 1 (b)] with
little or n o net s p a c e c h a r g e [Fig. 1 (a)]. A b o u t 5 x 1 0 1 3 /
c m 2 electrons a n d holes w e r e then alternately injected into
the V U V irradiated oxide. T h e C V results in Fig. 1 (a)
s h o w that, after the initial electron injection, the a m o u n t of
net trapped c h a r g e cycles with almost perfect repeatability
with a b o u t 1.5X lO I3 charges captured o n e a c h s u b s e q u e n t
injection. (Note that the positive shifts indicate electron
capture.)
Figure 1 (b) allows a c o m p a r i s o n of E S R E spin d e n sity data with C V c h a r g e density data in Fig. 1 (a). B e g i n n i n g with the initial electron injection, the E ’ m a g n i t u d e
cycles back a n d forth, c h a n g i n g a b o u t 8 X 1 0 ” spins p e r
cycle. This m a tches o u r C V data within a factor of two a n d
s h o w s that paramagnetic E ’ centers a r e capturing electrons
a n d diamagnetic 6 centers a r e capturing holes with a large
capture cross section ( - lo-l3 cm2). T h e fact that C V
m e a s u r e m e n t of s p a c e c h a r g e a n d E S R m e a s u r e m e n t s of
spin density d o not “m a tch” after the initial electron injection suggeststhat s o m e structural c h a n g e m a y b e occurring
at the trapping sites. T o determine whether or not this is
the case, w e explore the effects of electron a n d h o l e injection o n unilluminated ( n o V W ) oxides.
Figures 2 (a) a n d 2 (b) s h o w the effects of electron a n d
h o l e injection into unirradiated multiple implant b u r i e d
2890
Appl. Phys. Lett., Vol. 60, No. 23, 8 J u n e 1 9 9 2
oxides. N o paramagnetic signals could b e o b s e r v e d in the
oxides prior to c h a r g e injection. P h o toinjection of 5 x 1 0 1 3
electrons/cm* into a n unilluminated s a m p l e d o e s not g e n erate a m e a s u r a b l e E signal. C V m e a s u r e m e n t s indicate
virtually n o net s p a c e charge; very few electrons a r e
trapped. Injection of 5 X 1 0 1 3holes/cm* into the unirradiated b u r i e d oxide generates a fairly strong E ’ signal (5
X lO I spins/cm3) a n d a large ( - 1 7 0 V ) negative C V
shift. Electron injection d o e snot result in the generation of
paramagnetic E centers; h o l e injection d o e s g e n e r a t e E
centers. This strongly indicates that at least s o m e of the E
centers a r e positively c h a r g e d w h e n paramagnetic. T h e p e culiar line s h a p e [compare to Fig. 2 (c)] of the h o l e injection i n d u c e d E suggestsa different local environment than
that e x p e r i e n c e d by the “e x t e n d e d V U V ” g e n e r a t e d E ’.
Note also the a b s e n c eof electron trapping in the unirradiated oxide. In V U V irradiated oxides (Fig. 1 ), injection of
the s a m e n u m b e r of electrons resulted in a substantial
buildup of negative s p a c e charge. This indicates that V U V
irradiation causes s o m e sort of structural c h a n g e that results in a d e e p electron trap.
..~ ~ - ~
2 ””
‘---
.E
WI
‘C 2 0
-
-) 4 0 H O U R V U V
/
0
E
k 10
5
VI
_i-
-t
NO VUV
5.0
Electrons Injected ( 1 0 1 3 / c m 2 )
FIG. 3. S h o w n h e r e a r e C V shifts of a V U V irradiated oxide a n d a n
unirradiated oxide subjectedto similar electron injection. Substantialelectron trapping occurs in the W V irradiated s a m p l e s while little or n o
trapping occurs in the unirradiated sample.
Conley, Jr., L e n a h a n , a n d R o i t m a n
2890
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a) CV DATA
tiRE
El
Hl
E2
H2
5.0pFlEF!_.~
~~- ..-__ ~-.
-4 4.0[ EFG..vuv!
Al
“E
0
2
73
c
-W
3.0
2.0 1.0;
O.&-J
fl
_-.El
y
(~ Hl
. -..E2
I ~-H2
E3
FIG. 4. Effects of electron (E) and hole (H) injection on (a) space
charge density (CV) and (b) spin density (ESR) measurements of unilluminated buried oxides.
A comparison of irradiated and unirradiated oxides
demonstrates electron trap generation quite directly. Figure 3 allows a comparison of charge trapping in VUV illuminated and unilluminated multiple implant SIMOX oxides subjected to electron injection. Substantial electron
trapping ( > 10’2/cm2) occurs in the WV irradiated sample but not in the unirradiated sample. (Etchback studies
show the net negative charge to be distributed in an approximately uniform manner throughout the oxide.) VUV
illumination directly results in the creation of a deep electron trap. (This electron trapping phenomenon is different
from that recently reported by Boesch et al.,12 who demonstrated the presence of quite shallow electron traps in
SIMOX oxides.)
Figure 4 shows the effects of alternately injecting electrons and holes into unirradiated single implant oxides.
Once again, the charge cycles repeatably ( z 1.3 x 1013/cm2
charges/cycle) after the initial electron injection. However, the ESR results do not closely match the electrical
measurements. Although the amount of trapped charge
cycles repeatably, the number of paramagnetic E’ centers
grows by a substantial amount after each hole injection.
This strongly suggests structural change in the oxide.
In summary, we present evidence for structural change
and observe the creation of deep electron traps in SIMOX
buried oxides. We demonstrate that a significant fraction of
paramagnetic ,5’ centers are positively charged, although
some may be neutral. A negatively charged defect compensates for the positive charge in at least some E sites. Since
the appearance of deep electron traps coincides with the
appearance of paramagnetic E centers, a link between E
centers and the deep electron traps is strongly suggested.
Note added in proof. After submission of this letter, we
became aware of backgate threshold measurements of Ouisse et al. I3 on SIMOX transistors which suggest the presence of deep electron traps and a radiation induced enhancement in their density or cross section.
This work has been supported in part by the Defense
Nuclear Agency and the Office of Naval Research (Grant
No. NOOO14-89-J-2022).
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2891
Appl. Phys. Lett., Vol. 60, No. 23, 8 June 1992
2891
Conley, Jr., Lenahan, and Roitman
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